Inside CRT: Debug Heap Management

转自www.codeguru.com

When you compile a debug build of your program with Visual Studio and run it in debugger, you can see that the memory allocated or deallocated has funny values, such as 0xCDCDCDCD or 0xDDDDDDDD. This is the result of the work Microsoft has put in to detect memory corruption and leaks in the Win32 platform. In this article, I will explain how memory allocation/deallocation is done via new/delete or malloc/free.

First, I will explain what all these values that you see, like CD, DD, and so forth, mean.

ValueNameDescription0xCD

Clean Memory

Allocated memory via malloc or new but never written by the application.

0xDD

Dead Memory

Memory that has been released with delete or free. It is used to detect writing through dangling pointers.

0xFD

Fence Memory

Also known as "no mans land." This is used to wrap the allocated memory (like surrounding it with fences) and is used to detect indexing arrays out of bounds.

0xAB

(Allocated Block?)

Memory allocated by LocalAlloc().

0xBAADF00D

Bad Food

Memory allocated by LocalAlloc() with LMEM_FIXED, but not yet written to.

0xCC

When the code is compiled with the /GZ option, uninitialized variables are automatically assigned to this value (at byte level).

If you take a look at DBGHEAP.C, you can see how some of these values are defined:

static unsigned char _bNoMansLandFill = 0xFD; /* fill no-man's land with this */

static unsigned char _bDeadLandFill = 0xDD; /* fill free objects with this */

static unsigned char _bCleanLandFill = 0xCD; /* fill new objects with this */

Before going any further, take a look at the memory management function that I will refer in this article.

FunctionDescriptionmalloc

C/C++ function that allocates a block of memory from the heap. The implementation of the C++ operator new is based on malloc.

_malloc_dbg

Debug version of malloc; only available in the debug versions of the run-time libraries. _malloc_dbg is a debug version of the malloc function. When _DEBUG is not defined, each call to _malloc_dbg is reduced to a call to malloc. Both malloc and _malloc_dbg allocate a block of memory in the base heap, but _malloc_dbg offers several debugging features: buffers on either side of the user portion of the block to test for leaks, a block type parameter to track specific allocation types, and filename/linenumber information to determine the origin of allocation requests.

free

C/C++ function that frees an allocated block. The implementation of C++ operator delete is based on free.

_free_dbg

Debug version of free; only available in the debug versions of the run-time libraries. The _free_dbg function is a debug version of the free function. When _DEBUG is not defined, each call to _free_dbg is reduced to a call to free. Both free and _free_dbg free a memory block in the base heap, but _free_dbg accommodates two debugging features: the ability to keep freed blocks in the heap's linked list to simulate low memory conditions and a block type parameter to free specific allocation types.

LocalAlloc

GlobalAlloc

Win32 API to allocate the specified number of bytes from the heap. Windows memory management does not provide a separate local heap and global heap.

LocalFree

GlobalFree

Win32 API free the specified local memory object and invalidates its handle.

HeapAlloc

Win32 API allocates a block of memory from a heap. The allocated memory is not movable.

HeapFree

Win32 API frees a memory block allocated from a heap by the HeapAlloc or HeapReAlloc function.

There are many other functions that deal with memory management. For a complete view please refer to MSDN.

Note: Because this article is about memory management in a debug build, all the references to malloc and free in the following are actually references to their debug versions, _malloc_dbg and _free_dbg.Compile the following code and run it in the debugger, walking step by step into it to see how memory is allocated and deallocated.

int main(int argc, char* argv[])

{

char *buffer = new char[12];

delete [] buffer;

return 0;

}

Here, 12 bytes are dynamically allocated, but the CRT allocates more than that by wrapping the allocated block with bookkeeping information. For each allocated block, the CRT keeps information in a structure called _CrtMemBlockHeader, which is declared in DBGINT.H:

#define nNoMansLandSize 4

typedef struct _CrtMemBlockHeader

{

struct _CrtMemBlockHeader * pBlockHeaderNext;

struct _CrtMemBlockHeader * pBlockHeaderPrev;

char * szFileName;

int nLine;

size_t nDataSize;

int nBlockUse;

long lRequest;

unsigned char gap[nNoMansLandSize];

/* followed by:

* unsigned char data[nDataSize];

* unsigned char anotherGap[nNoMansLandSize];

} _CrtMemBlockHeader;

It stores the following information:

FieldDescriptionpBlockHeaderNext

A pointer to the next block allocated, but next means the previous allocated block because the list is seen as a stack, with the latest allocated block at the top.

pBlockHeaderPrev

A pointer to the previous block allocated; this means the block that was allocated after the current block.

szFileName

A pointer to the name of the file in which the call to malloc was made, if known.

nLine

The line in the source file indicated by szFileName at which the call to malloc was made, if known.

nDataSize

Number of bytes requested

nBlockUse

0 - Freed block, but not released back to the Win32 heap

1 - Normal block (allocated with new/malloc)

2 - CRT blocks, allocated by CRT for its own use

lRequest

Counter incremented with each allocation

gap

A zone of 4 bytes (in the current implementation) filled with 0xFD, fencing the data block, of nDataSize bytes. Another block filled with 0xFD of the same size follows the data.

Most of the work of heap block allocation and deallocation are made by HeapAlloc() and HeapFree(). When you request 12 bytes to be allocated on the heap, malloc() will call HeapAlloc(), requesting 36 more bytes.

blockSize = sizeof(_CrtMemBlockHeader) + nSize + nNoMansLandSize;

malloc requests space for the 12 bytes we need (nSize), plus 32 bytes for the _CrtMemBlockHeader structure and another nNoMansLandSize bytes (4 bytes) to fence the data zone and close the gap.

But, HeapAlloc() will allocate even more bytes: 8 bytes below the requested block (that is, at a lower address) and 32 above it (that is, at a bigger address). It also initializes the requested block to 0xBAADF00D (bad food).

Then, malloc() fills the _CrtMemBlockHeader block with information and initializes the data block with 0xCD and no mans land with 0xFD.

Here is a table that shows how memory looks after the call to HeapAlloc() and after malloc() returns. For a complete situation, see the last table. (Note: All values are in hex.)

Addressafter HeapAlloc()after malloc()00320FD8

00320FDC

00320FE0

00320FE4

00320FE8

00320FEC

00320FF0

00320FF4

00320FF8

00320FFC

00321000

00321004

00321008

0032100C

00321010

00321014

00321018

0032101C

00321020

00321024

00321028

0032102C

09 00 09 01

E8 07 18 00

0D F0 AD BA

AB AB AB AB

00 00 00 00

79 00 09 00

EE 04 EE 00

40 05 32 00

09 00 09 01

E8 07 18 00

98 07 32 00

00 00 00 00

0C 00 00 00

01 00 00 00

2E 00 00 00

FD FD FD FD

CD CD CD CD

FD FD FD FD

AB AB AB AB

00 00 00 00

79 00 09 00

EE 04 EE 00

40 05 32 00

Colors:

Green: win32 bookkeeping info Blue: block size requested by malloc and filled with bad food Magenta: _CrtMemBlockHeader block Red: no mans land Black: requested data block In this example, after the call to malloc() returns, buffer will point to memory address 0x00321000.

When you call delete/free, the CRT will set the block it requested from HeapAlloc() to 0xDD, indicating this is a free zone. Normally after this, free() will call HeapFree() to give back the block to the Win32 heap, in which case the block will be overwritten with 0xFEEEEEEE, to indicate Win32 heap free memory.

You can avoid this by using the CRTDBG_DELAY_FREE_MEM_DF flag to _CrtSetDbgFlag(). It prevents memory from actually being freed, as for simulating low-memory conditions. When this bit is on, freed blocks are kept in the debug heap's linked list but are marked as _FREE_BLOCK. This is useful if you want to detect dangling pointers errors, which can be done by verifying if the freed block is written with 0xDD pattern or something else. Use _CrtCheckMemory() to verify the heap.s integrity.

The next table shows how the memory looks during the free(), before HeapFree() is called and afterwards.

AddressBefore HeapFree()After HeapFree()00320FD8

00320FDC

00320FE0

00320FE4

00320FE8

00320FEC

00320FF0

00320FF4

00320FF8

00320FFC

00321000

00321004

00321008

0032100C

00321010

00321014

00321018

0032101C

00321020

00321024

00321028

0032102C

09 00 09 01

5E 07 18 00

DD DD DD DD

AB AB AB AB

00 00 00 00

79 00 09 00

EE 04 EE 00

40 05 32 00

82 00 09 01

5E 04 18 00

E0 2B 32 00

78 01 32 00

EE FE EE FE

Colors:

Green: win32 bookkeeping info Blue: CRT block filled with dead memory Gray: memory given back to win32 heap The two tables above are put in a single, more detailed, table below:

Address (hex)OffsetHeapAllocmallocFree before HeapFreeFree after HeapFreeDescription00320FD8

-40

01090009

01090082

Win32 Heap info

00320FDC

-36

001807E8

0018075E

0018045E

Win32 Heap info

00320FE0

-32

BAADF00D

00320798

DDDDDDDD

00322BE0

pBlockHeaderNext

00320FE4

-28

BAADF00D

00000000

DDDDDDDD

00320178

pBlockHeaderPrev

00320FE8

-24

BAADF00D

00000000

DDDDDDDD

FEEEEEEE

szFileName

00320FEC

-20

BAADF00D

00000000

DDDDDDDD

FEEEEEEE

nLine

00320FF0

-16

BAADF00D

0000000C

DDDDDDDD

FEEEEEEE

nDataSize

00320FF4

-12

BAADF00D

00000001

DDDDDDDD

FEEEEEEE

nBlockUse

00320FF8

-8

BAADF00D

0000002E

DDDDDDDD

FEEEEEEE

lRequest

00320FFC

-4

BAADF00D

FDFDFDFD

DDDDDDDD

FEEEEEEE

gap (no mans land)

00321000

BAADF00D

CDCDCDCD

DDDDDDDD

FEEEEEEE

Data requested

00321004

BAADF00D

CDCDCDCD

DDDDDDDD

FEEEEEEE

Data requested

00321008

BAADF00D

CDCDCDCD

DDDDDDDD

FEEEEEEE

Data requested

0032100C

+12

BAADF00D

FDFDFDFD

DDDDDDDD

FEEEEEEE

No mans land

00321010

+16

ABABABAB

FEEEEEEE

Win32 Heap info

00321014

+20

ABABABAB

FEEEEEEE

Win32 Heap info

00321018

+24

00000000

FEEEEEEE

Win32 Heap info

0032101C

+28

00000000

FEEEEEEE

Win32 Heap info

00321020

+32

00090079

FEEEEEEE

Win32 Heap info

00321024

+36

00EE04EE

FEEEEEEE

Win32 Heap info

00321028

+40

00320540

FEEEEEEE

Win32 Heap info

0032102C

+44

00320540

FEEEEEEE

Win32 Heap info

About the Author

Marius Bancila is a software developer working for a company that provides industrial automation solutions, but he is mainly focused on GUI design with MFC. When he discovered the .NET framework quickly fell inlove. He considers that CodeGuru is the best place on internet to spend time on.